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From IRPS Bulletin Vol 10 No 3 September/October, 1996
ONE HUNDRED YEARS OF ELECTRONS
Leif Gerwarda and Christopher Cousinsb
a. Department of Physics, Technical University of
Denmark, Lyngby, Denmark
b. Department of Physics, University of Exeter,
Exeter, UK
The search for the true nature of electricity led
to the discovery of the electron and the proof
that it is a constituent of all atoms. These
achievements gave the scientists the first
definite line of attack on the constitution of the
atom and on the structure of matter.
In a lecture on 'The Stability of Atoms' given to the Royal
Society of Arts in 1924, Ernest Rutherford said:
The trend of physics during the past twenty-five years has
been largely influenced by three fundamental discoveries
made in the closing years of the nineteenth century. I
refer to the discovery of X-rays in 1895, of radioactivity
in 1896 and the proof of the independent existence of the
negative electron of small mass in 1897.
The first two of these discoveries were revelations of
totally unsuspected phenomena, rare events in the
progress of science. The third was quite different - it was
a watershed in the long quest to understand the nature of
electricity. In the present paper we focus on the period of
activity concerned with discharge in gases and cathode
rays which culminated in the discovery of the electron.
Finally and briefly we comment on some of the
developments of the concept during the following
hundred years.
Already in 1874, at the British Association meeting in
Belfast, the Irish physicist George Johnstone Stoney had
pointed out that, on the basis of Faraday's law of
electricity, an absolute unit of quantity of electricity
exists in that amount of it which attends each chemical
bond or valency. This talk was later published (Stoney
1881). Stoney subsequently suggested the name electron
for this small unit quantity of electricity (J.J. 1911/12).
Hermann von Helmholtz (1881), independently, drew
similar conclusions. In the Faraday lecture delivered
before the Chemical Society in 1881 he declared:
If we accept the hypothesis that the elementary
substances are composed of atoms, we cannot avoid
concluding that electricity also, positive as well as
negative, is divided into definite elementary portions
which behave like atoms of electricity.
The discovery of the cathode rays by Wilhelm Hittorf in
1869 forms the first link in a chain of events that leads
not only to the triumphant confirmation of the
above hypothesis but also to the brilliant discoveries
made in the closing years of the 19th century: that of Xrays by Wilhelm Conrad Röntgen in 1895 and that of
radioactivity by Henri Becquerel in 1896.
Cathode rays occurs when electricity is discharged in a
rarefied gas. At a given low pressure of the gas, rays are
emitted from the negative pole, the cathode. The
cathode rays themselves are invisible, but they can be
observed by the phosphorescence that they induce at the
walls of the glass tube and by the shadows from objects
placed in their path. The cathode rays propagate in
straight lines, but unlike ordinary light rays they can be
deflected by a magnetic field.
The phenomena exhibited by the electric discharge in
rarefied gas had long been familiar from studies by
Julius Plücker, Wilhelm Hittorf, Eugen Goldstein and
other physicists. In a lecture delivered before the British
Association at the Sheffield Meeting in 1879, William
Crookes first used the expression radiant matter or
matter in the ultra-gaseous state, to explain the
phenomena of phosphorescence, trajectory, shadows,
mechanical action, magnetisation and heat associated
with the cathode rays (Crookes 1901/02). Crookes
considered the ultra-gaseous state a 'fourth state of
matter'. In the Bakerian Lecture for 1883, speaking of
radiant matter, he considered that the particles flying
from the the cathode were of the dimensions of
molecules (W.A.T. 1919/20). Following Crookes many
British physicists held that the cathode rays were
particles. In contrast, the majority of the German
physicists maintained that the cathode rays were
analogous to electric waves. They were inspired by
Heinrich Hertz (1892) who, in 1892, had shown that the
cathode rays could pass through thin sheets of metal. At
Hertz' suggestion, Philipp Lenard made a kind of
discharge tubes equipped with a thin aluminium
window, allowing the cathode rays to enter the open air
outside the tube.
At the end of 1884, when the English physicist Joseph
John (J.J.) Thomson was not quite 28 years old, he
succeeded Lord Rayleigh as professor and director of
the Cavendish Laboratory in Cambridge. He
immediately began experiments on the passage of
electricity through gases. He was convinced, he later
said, that "whenever a gas conducts electricity, some of
its molecules must split up and that it is these particles
which carry electricity" (Thomson 1926). Originally, he
thought that the molecule was split up into two atoms. It
was not until 1897 that he was to realize that the At the
beginning of 1897 Thomson made some experiments to
clarify the nature of the cathode rays, in particular to test
some consequences of the particle theory (Thomson
1897). He first repeated, in a modified form, the
experiment of Jean Perrin of Lille, France, to show that
the cathode rays carry a negative charge.
Next, he measured the deflection of the cathode rays
under an electrostatic force, the rays travelling between
two parallel and electrically charged aluminium plates.
Hertz had been unable to detect any deflection in a
similar experiment, but Thomson showed that this was
due to the conductivity conferred on the rarefied gas by
the cathode rays. The deflection could indeed be
detected, even in a weak electric field, provided that the
vacuum was good enough. Moreover, he found that the
deflection was proportional to the potential difference
between the plates. Finally he measured the deflection of
the cathode rays under a magnetic field. He concluded
(Thomson 1897):
Commenting on his talk not three weeks later George
Francis FitzGerald (1897) suggested that
“We are dealing with free electrons in these cathode
rays.”
As the cathode rays carry a charge of negative electricity,
are deflected by an electrostatic force as if they were
negatively electrified, and are acted on by a magnetic
force in just the way in which this force would act on a
negatively electrified body moving along the path of these
rays, I can see no escape from the conclusion that they are
charges of negative electricity carried by particles of
matter.
To find out whether these particles are molecules, atoms,
or matter in a still finer division, Thomson further made a
series of measurements of the ratio of the mass, m, of each
of these particles to the charge, e, carried by it. He
devised two methods for deflecting the cathode rays, one
in a magnetic field, the other in an electric field. In both
cases he found that the value of m/e is independent of the
nature of the gas in the discharge tube, and its value is of
the order 1/1000 of the smallest value known at that time,
namely the value for the hydrogen ion in electrolysis.
Thomson speculated that the smallness of m/e might be
due to the smallness of m, or the largeness of e, or to a
combination of these two. A support for the smallness of
m was delivered by Lenard, who had measured the
absorption of the cathode rays by various media. From
Lenard's result the mean free path in air at atmospheric
pressure could be estimated to be 0.5 cm. This value is far
larger than the value for the mean free path of the air
molecules themselves, showing that the cathode ray
particles must be small compared with ordinary
molecules.
From his experiments Thomson concluded that, in the
intense electric field near the cathode, the molecules of the
gas are split up, not into ordinary atoms but into particles
of a primordinal element, which he called corpuscles.
These corpuscles, which are charged with negative
electricity and projected from the cathode by the electric
field, constitute the cathode rays. According to Thomson
(1897) cathode rays represent matter in a new state, a state
in which the subdivision of matter is carried very much
further than in the ordinary gaseous state: a state in which
all matter - that is, matter derived from different sources
such as hydrogen, oxygen, &c. - is of one and the same
kind; this matter being the substance from which all the
chemical elements are built up.
Thomson gave his first public account of his discovery
that in cathode rays there are particles far more minute
than anything previously recognized at the Friday
meeting of the Royal Institution on 30th April 1897.
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There is a delightful aptness in this identification, for
FitzGerald was related to Stoney, being both his nephew
and his cousin! Though Thomson was the undisputed
discoverer of the electron, there were others in the hunt
who came close to the prize. In particular, Perrin (1965)
relates in his 1926 Nobel lecture how he had already
begun work involving electric and magnetic deflection
of cathode rays when Thomson's 1897 paper appeared.
In his 1897 paper, Thomson tended to consider the
smallness of m/e as due to a combination of the
smallness of m and the largeness of e. In subsequent
work (Thomson 1899) he devised a method for direct
measurements of e as well as m/e, thus allowing the
mass of the particles to be determined. The
measurements showed that e is the same in magnitude as
the charge carried by the hydrogen ion in the electrolysis
of solutions. Thus Thomson had proved that the mass of
the carriers of negative electricity is of the order of
1/1000 of that of the hydrogen atom, the smallest mass
known at that time. This numerical result was perfectly
adequate for the interpretation adopted and, if not at first
very accurate, was soon improved by later experiments.
By way of contrast, it followed from experiments by
Wilhelm Wien on the ratio of the mass to the electric
charge for carriers of positive electrification that these
masses were comparable with those of ordinary atoms.
The method for measuring the charge of the ion was
based on the discovery by Charles Thomson Rees
(C.T.R.) Wilson that the ions form nuclei around which
water will condense from dust-free air at a certain
supersaturation. The problem of finding the number of
ions per volume was now reduced to find the number of
drops per volume in the cloud. This was done by
observing the velocity with which the drops fall under
gravity and by deducing the radius of each drop from a
formula containing the viscosity of the gas through
which the drop falls. Finally, it was necessary to know
the velocity with which the ion moves under a known
electric force. This parameter was determined by the
New Zealand-born English physicist Ernest Rutherford,
who had arrived at Cambridge in October 1895 to carry
out research at Cavendish Laboratory under Thomson's
guidance.
Thomson concluded that the negative charge carrier, its
mass and charge being invariable, must represent a
fundamental conception in electricity, or indeed in
matter in any state. He proposed to regard the atom as
containing a large number of smaller bodies, which he
called corpuscles. With regard to their size he estimated
what would come to be known as the 'classical electron
radius' to be of the order of 10-13 cm. In the normal
atom, the assembly of corpuscles forms a system which
is electical neutral. Detached corpuscles behave like
negative ions, each carrying a constant negative charge
and having a very small mass.
September/October 1996
The atom left behind behaves like a positive ion with a
mass much larger than the negative ion. In short, the
corpuscles are the vehicles by which electricity is
carried from one atom to another. Thomson was
convinced that there are more corpuscles in the atom
than the one or two that could be torn off by processes
known at that time. A strong support for this hypothesis
was afforded by the Zeeman effect, which accordingly
affects a large number of lines in the spectrum from a
given atom.
principle drove the development of Fermi-Dirac statistics
to the puzzling shortfall in the thermal capacity of metals.
Through Thomson's discovery of the electron, numerous
strange effects observed in the past got their natural
explanation. In a paper read before the Royal Society on
6th February 1902, Sir William Crookes (1901/02)
summarized:
Nearly twenty-five years ago I was led by experiments in
highly rarefied tubes to assume the existence of matter in
an ultra-gaseous state. /.../ What I then called "Radiant
Matter" now passes as "Electrons", a term coined by Dr.
Johnstone Stoney, to represent the separate units of
electricity, which is as atomic as matter. What was
puzzling and unexplained on the "Radiant Matter" theory
is now precise and luminous on the "Electron Theory".
/.../ The electrons are the same as the "satellites" of Lord
Kelvin and the "corpuscles" of J.J. Thomson.
The early years of the century brought the discovery of
superconductivity in metals, more recently that of highTc superconductivity in certain cuprates. In the closing
years we find that technology has reached such a pitch
of delicacy that 'artificial atoms' can be constructed:
nanometer-sized metal or semiconductor systems with a
few electrons designed to function as transistors or other
devices.
Thus Thomson's discovery of the electron not only
produced the explanation of a number of puzzling
observations in the past but also opened up new fields of
science. Of these the richest is surely that of the
electronic structure of matter, a field of unceasing
development, which has given us, amongst many other
things, the electronic chip, the computer, and,
ultimately, an unprecedented power of global
intercommunication.
References
Bohr, N. (1926). Sir J.J. Thomson's Seventieth Birthday.
Nature, December 18, p. 879.
Crookes, W. (1901/02). The Stratifications of
Hydrogen. Proc. Roy. Soc. (London) 69, 399-413.
J.J. Thomson received the Nobel Prize in physics in 1906
in recognition of the great merits of his theoretical and
experimental investigations on the conductivity of
electricity by gases, and he was knighted in 1908. At his
seventieth birthday, 18th December 1926, he received
numerous messages of congratulation from abroad. Niels
Bohr, professor of the Institute of Theoretical Physics in
Copenhagen wrote (Bohr 1926):
FitzGerald, G.F. (1897). Electrician 39, 103.
Guided by his wonderful imagination and leaning on the
new discoveries of the cathode rays, Röntgen rays and
radioactivity, he opened up an unknown land to science.
Indeed, it is difficult for scientists of the younger
generation, who are working on the new land to which
Thomson has opened the gates, fully to realize the
magnitude of the task with which the pioneers were
confronted.
J.J. (1911/12). George Johnstone Stoney, 1812-1911.
Proc. Roy. Soc. (London) 86, xx-xxxv.
The growth in the understanding of the nature of
electricity was a triumph of human curiosity, competition
and cooperation. In the electron we recognize the very
first of the many fundamental particles that have been
proposed during this last hundred years, and it has
survived where others have perished.
Thomson, J.J. (1897). Cathode Rays. Phil. Mag. 44,
293-316.
Advances continue, and the electron of today is not the
simple entity it was: it has acquired angular momentum; it
has been drawn into the quantum world where its
behaviour is brilliantly captured, first by the Schrödinger
and Dirac equations and later by quantum
electrodynamics; it has been joined by a positive
counterpart, the positron. Beams of electrons possess
wavelike properties, and electron diffraction joins X-ray
diffraction as a powerful probe of the structure of matter.
Through the Pauli exclusion principle we understand the
Periodic Table and make sense of atomic and molecular
spectra, and the characteristic X-radiation. The same
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Helmholtz, H. (1881). On the modern development of
Faraday's conceptions of electricity. J. Chem. Soc.
39, 277-304.
Hertz, H. (1892). Über den Durchgang von
Kathodenstrahlen durch dünne Metallschichten.
Annal. Phys. Chem. 45, 28-32.
Perrin, J. (1965). Discontinuous structure of matter. In
Nobel Lectures, Physics 1922-1941. The Nobel
Foundation. Elsevier Publ. Co., Amsterdam, pp. 138164.
Stoney, G.J. (1881). Phil. Mag. 11, 381.
Thomson, J.J. (1899). On the Masses of the Ions in
Gases at Low Pressures. Phil. Mag. 48, 547-567.
Thomson, J.J. (1926). Retrospect. Nature, December 18,
Supplement, pp. 41-44.
W.A.T. (1919/20). Sir William Crookes, O.M., 18321919. Proc. Roy. Soc. (London) A 96, i-ix.
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